(1) Field of the Invention
The present invention relates to a secondary air supply device which can be connected to an exhaust pipe of an internal combustion engine to introduce or add fresh air into the exhaust gas to after-burn the unburned combustibles in the exhaust gas.
(2) Description of the Prior Art
It is well known in the art to add a secondary air into the stream of exhaust gas flowing through an exhaust pipe of an internal combustion engine in an attempt to reduce exhaust emissions. Oxygen in the secondary air reacts with hot exhaust gas to convert unburned combustibles such as HC and CO into harmless gases. Introduction of the secondary air is achieved by means of a secondary air supply system including a supply conduit connected at one end to the exhaust pipe. The inlet end of the supply conduit is generally connected to an air cleaner of the combustible mixture supply system of the enginr. A check valve such as a reed valve is placed in the supply conduit. The exhaust gas pressure at the juncture of the supply conduit with the exhaust pipe varies cyclically between a positive and a negative pressure in respones to the opening of the exhaust valve. Note, the term positive pressure as used herein is defined as a pressure higher than ambient pressure, and the term negative pressure is defined as a pressure lower than abmient pressure. The pressure variation at the juncture, in turn, causes pressure pulsations in the air in the secondary air supply conduit. As the pressure in the supply conduit pulsates, the secondary air is drawn through the secondary air supply conduit into the exhaust pipe during the negative pressure period. During the positive pressure period, the check valve prevents backflow of the exhaust gas out of the inlet end of the secondary air supply conduit.
Where the inlet end of the secondary air supply conduit is to be connected to the air cleaner, the supply conduit must have a considerable length. In other instances, it is desirable to locate the inlet end of the supply conduit at a cool air region remote from the exhaust pipe, in order to protect the reed valve from the heat issuing from the engine. In both cases, the requirement is for increasing the length of the secondary air supply conduit.
The frequency of resonance f of the secondary air supply conduit and the length l thereof have a correlation expressed by the equation ##EQU1## wherein c is the velocity of sound. Thus, the frequency of resonance of the supply conduit decreases as the length increases. This results in the exhaust gas pressure pulsating only in the positive pressure range when the engine is rotating at a higher speed, whereby the secondary air supply system fails to permit the fresh air to flow into the exhaust pipe.